Method for producing smooth galvanized sheet



w. C.S|EVERT 3,323,940

METHOD FOR PRODUCING SMOOTH GALVANIZED SHEET June 6, 1967 Filed Jan 20,1964 N M U United States Patent 3,323,940 METHOD FOR PRODUCING SMOOTHGALVANIZED SHEET William C. Sievert, Chesterton, Ind., assignor toInland Steel Company, Chicago, 111., a corporation of Delaware FiledJan. 20, 1964, Ser. No. 338,860 3 Claims. (Cl. 117114) The presentinvention relates generally to processes for galvanizing flat, ferrousbase material, and more particularly to a process forproducinggalvanized, flat, ferrous base material having a smooth,non-spangled, light gray, readily paintable surface coating primarily ofzinc and having an iron content which is uniform along the entire areaof the coated surface.

The production of' smooth, readily paintable, nonspangled, galvanizedstrip having a surface coating of zinc alloyed with iron has beendescribed generally in Mehler et al. US. Letters Patent No. 3,056,694.It is desirable thatthe iron content in the coating be uniform along theentire area of a coated surface on a strip and that this uniformity beconsistent from strip to strip. The process of the present inventionproduces a coating of the type described generally in Mehler, and inaddition produces a coating which is uniform from one end of acontinuous strip to the other, and from one side edge of the strip tothe other, as well as from continuous strip to continuous strip, nomatter at what date the strip is made.

In addition to optimizing coating uniformity, the process of thepresent-invention also imparts economies and minimizes the investment inequipment to an extent not available with piocesses'lacking the featuresof the present invention.

Other features and advantages are inherent in'the process claimed anddisclosed or will becomeapparent to those skilled in the art fromthefollowing detailed description in conjunction with the accompanyingdiagrammatic drawing wherein:

FIGURE 1 is a schematic representation of apparatus suitable forcarrying out an embodiment of a process in accordance with the presentinvention; and

FIGURE 2 is a plan View of flat, ferrous base galvanized material.

Basically, a strip to be treated in accordance with the presentinvention is initially coated, preferably in the manner described indetail in. US; Letters Patent No. 2,110,898, by a process conventionallyknown as the Sendzimir type of process. After coating with zinc, thestrip is subjected to a post heating or galvannealing step whichpromotes diffusion of iron from the ferrous base material into the zinccoating. As a result of this post heating step, the galvanized coatingchanges its appearance from a bright, spangled one to a non-spangled,smooth, light gray appearance. A product of this type, and with auniform coating having an iron content between 4 and 7 Wt. percent ofthe final coating, is the end result of a process practiced inaccordance with the present invention, and now to be described indetail.

Initially, there is provided a coil of ferrous base flat material, suchas steel strip, having a composition of the type conventionally used forgalvanizing purposes (e.g., see the compositions listed on pages 666-7in The Making, Shaping and Treating of Steel, 1957, United States SteelCorp., Pittsburgh, Pa.). The thickness of the base material may be anyone of the conventional thicknesses 3,323,940 Patented June 6, 1967 icenormally utilized for producing galvanized flat material (e.g., 10-24gauge).

Referring to FIGURE 1, a steel strip 12 from a coil 11 is fed through anoxidizing furnace 13 in which the strip 12 is heated under oxidizingconditions to produce a thin, uniform oxide coating on the strip. Thestrip then passes through a furnace 14 containing a reducing atmospherein which the oxide coating produced in furnace 13 is reduced to atightly adhering layer of base metal free of oxides and otherimpurities. From reducing furnace 14 the strip passes over a roller 15and into a hood or spout 16 leading from reducing furnace 14 to a pointbelow the surface of a bath of molten zinc 17 contained in a pot 25. Themolten zinc bath contains 0.05-030 wt. percent aluminum, with thepreferable amount of aluminum being 0.13-0.18 wt. percent.

Spout 16, by extending below the surface of the molten zinc 17, forms aneffective seal against the entry into reducing furnace 14 of oxidizinggases located above the surface of the molten zinc 17.

An important feature of the present invention is that the immersiontemperature of strip 12, i.e., the temperature at which the strip entersthe pot of molten zinc 17, is no greater than 900 F. An immersiontemperature over 900 F. will over-promote diffusion of iron from theferrous base into the zinc coating to such an extent that the final ironcontent in the zinc coating cannot be controlled to the degree necessaryto produce the desirable light gray, non-spangled, smooth, adheringcoating. A strip immersion temperature greater than 900 F. can causeoveralloying to occur to such an extent that a dark gray, brittlecoating will occur on the ferrous base strip, and the material will beunsuitable for bending or other deforming purposes.

There is no specified minimum immersion temperature for the strip, asthere is a maximum temperature, although the strip will be at leastabove room temperature inasmuch as it has passed through two furnaces13, 14 before entering zinc pot 25.

The purpose of the aluminum in the molten zinc is to retard or slow downdiffusion of iron from the ferrous base into the zinc coating so thatcontrol can be exercised over the proportion of iron in the coating. Asused herein, the term traces of aluminum means an amount of aluminumsufiicient to enable satisfactory control to be exercised within theother condtions prevailing during the process. As indicated above, ithas been conventional to utilize 0.05-0.30 wt. percent aluminum in themolten zinc, with a preferable aluminum content being 0.13- 0.18 wt.percent. Both of these ranges are contemplated within the term traces ofaluminum.

Following immersion of strip 12 in zinc pot 25, the strip passes arounda sinker roll 19, and then out of zinc bath 17 and through coating rolls20 partially immersed in bath 17 and which are used to control thethickness of the zinc coating on the strip. conventionally, the weightof the zinc coating on a single given surface of galvanized, flatferrous base material is 0.1-1.0 ounce per square foot. In a typicalgalvanizing operation, a coating on one surface might be about 0.4-0.5ounce per square foot for the one surface. Oftentimes, the coatingweight on one surface differs from the coating weight on the othersurface, depending upon the purpose for which the strip is to beutilized. In contrast, in a process in accordance with the presentinvention the coating weights must be controlled so that they are thesame on both surfaces of the strip.

In the process of the present invention, not only is the weight of thezinc coating controlled generally, but also, an important feature is thecontrol of variations in coating weight from the front edge 26 to therear edge 27 of strip 12, as well as of variations in coating Weightfrom one side edge 28 to another side edge 29 (FIG. 2). In accordancewith the present invention, the variation should not exceed $0.05 ounceper square foot on a given surface. Otherwise there will be eitherunderalloying or overalloying, as well as the correct amount ofalloying, at various localized areas on any given surface of any givenstrip. Such variations are undesirable because they result infabricating difficulties or early failures of a localized portion of anobject made out of the strip. Because of the step of controlling thevariation in coating weight Within :05 ounce per square foot, there areno such localized areas of underalloying or overalloying on a surface offiat galvanized material produced in accordance with the presentinvention.

Variations in coating weight along a given surface may be controlled byemploying a number of techniques familiar to those skilled in the artand including: adjusting the tension of the coating rolls 20 on thestrip; adjusting the depth of the coating rolls in the zinc bath;changing the number and depth of grooves with which the coating rollsare conventionally provided; maintaining the grooves free ofaccumulations of zinc dross; and adjusting the temperature of the zincbath.

After coating, the next step in the process is to move the strip throughalloying furnace 21 which may be fired in any conventional manner. Animportant feature of the present invention is that the furnace provide atemperature and have a heating capacity such that the strip is heated,at a strip heating rate of 2530 F. per second, to

a maximum strip temperature between 1000 and 1100 F., to promotediffusion of iron from the ferrous base strip into the coating. Withthis combination of heating rate and maximum strip temperature, theheating operation can be performed without holding the strip for any'present invention are being practiced; and it is important to practiceall of these conditions in order to obtain the desired economies ofoperation and equipment investment.

Immediately upon attainment of the maximum strip temperature (l0001l00F.), the strip is cooled, without holding at the maximum striptemperature, to a strip temperature of 800 F. The cooling rate iscontrolled, between the maximum strip temperature and 800 F., todiflfuse iron from the flat ferrous base strip'into the coating untilthe amount of iron in the coating is between 4 and 7 wt. percent of thefinal coating. Although, in accordance with the present invention, thestrip immersion temperature, the heating rate and the maximum striptemperature may be essentially the same from strip to strip, the desiredthickness of the coating will vary, so that the actual cooling rate willvary with the thickness of the zinc coating. For example, assuming acoating thickness of 0.40.5 ounce of zinc per square foot on onesurface, the cooling rate between the maximum strip temperature and 800F. should be between 12 and 14 F. per second. With a thicker coating thecooling rate would be slower, and with a thinner coating the coolingrate would be faster. However, by assuring that the immersiontemperature of the ferrous strip is below 900 F., and by assuring aheating rate of 2530 F. per second and a maximum strip temperature ofI000'1l O0' F., the desired amount of iron in the zinc coating (between4 and 7 wt. percent) can be obtained for virtually any thickness ofcoating within the thickness range conventionally utilized on galvanizedflat base material (O.ll.0 ounce per square foot on a given surface),merely by cooling to 800 F. using a cooling rate which is commerciallypracticable.

After the strip has been cooled from the maximum strip temperature to800 F., utilizing a cooling rate to produce an iron content in thecoating between 4 and 7 wt. percent, the strip is cooled rapidly from800 F. at a rate sufficiently rapid to prevent further diffusion of ironinto the coating. Once the temperature of the strip is cooledsubstantially below 800 F., little further diffusion of iron will occur.Therefore, it is important that the temperature be rapidly reduced below800 F. immediately upon attainment of the desired amount of iron in thecoating. This rapid cooling may be performed utilizing a stream of airor water directed against the coating or by any other method which willreduce the temperature of the strip rapidly enough to prevent furtherdiffusion of iron.

The cooling operations may be performed in one or more cooling chambers22 in which the strip is cooled by means such as a stream of cold air ora fine spray of water. The strip then passes over a roller 23 andsubsequently typically proceeds to a coiling operation.

By utilizing a cooling operation of the type described,

wherein the strip cooling rate is controlled down to 800 F. so as topromote diffusion of iron, and then the strip is cooled rapidly once itreaches 800 F. to prevent diffusion of iron; and by utilizing a stripimmersion temperature, a strip heating rate and a maximum striptemperature within the respective ranges described above, the necessityof holding the strip at maximum strip temperature is eliminated. Becausethe strip need not be held for any length of time at the maximum striptemperature, it is not necessary to provide long, expensive furnaces.

Thus, in accordance with they present invention, there is produced asmooth, non-Spangled, light gray galvanized coating on a flat ferrousbasev strip; and this coating is readily paintable, and has a uniformiron content from front edge to rear edge of any given strip, as well asfrom side edge to side edge of any given strip. In addition, the coatingis produced utilizing a process which is economical, efiicient andreadily controllable.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom, as modifications will be obvious to those skilled in the art.

What is claimed is;

1. A process for producing flat, galvanized ferrous strip having auniform, smooth, non-Spangled, light gray surface coating, said processcomprising the steps of:

immersing a flat ferrous base strip, with each surface free of oxidesand other impurities and having a strip temperature below 900 F., into abath of molten zinc containing traces of aluminum;

coating each surface of said base strip with the same thickness of saidzinc, between 0.1 and 1.0 ounce per square foot of said zinc;

controlling the variation in said zinc coating on a surface, from sideto side and from front edge to rear edge of the strip, to 1.05 ounce persquare foot;

heating the coated strip, at a strip heating rate of 25 30 F. persecond, to a maximum strip temperature between 10001100 F. to promotethe diffusion of iron from said ferrous base strip into said coating;

cooling said strip immediately upon attainment of said maximum striptemperature, without holding at said maximum st-ri-p temperature, to astrip temperature of 800 F.;

controlling the cooling r ate,between said maximum strip temperaturreand 800 F., to diffuse iron from said fiat ferrous base strip into thecoating in an amount of iron between 4 and 7% of the final coatingWeight;

and then cooling said strip upon attainment of said 800 F. to atemperature below 800 F. at a cooling rate sufiicient to prevent furtherdiffusion of iron into said coating.

2. A process as recited in claim 1 wherein:

said strip is coated on each surface thereof with 4-5 ounce of zinc persquare foot;

and said coated strip is cooled from said maximum temperature to 800 F.,at a cooling rate of 12- 14 F. per second.

6 3. A process as recited in claim 1 wherein said cooling rate from 800F. to a temperature below 800 F. is faster than said cooling rate fromsaid maximum strip temperature to 800 F.

References Cited UNITED STATES PATENTS 3,056,694 10/1962 Mehler et al.117114 3,058,840 10/1962 Kerr et al 1171 14 X 3,177,088 4/1965 Sievert117-114 ALFRED L. LEAVI'IT, Primary Examiner.

R. S. KENDALL, Assistant Examiner.

1. A PROCESS FOR PRODUCING FLAT GALVANIZED FERROUS STRIP HAVING A UNIFORM, SMOOTH, NON-SPANGLED, LIGHT GRAY SURFACE COATING, SAID PROCESS COMPRISING THE STEPS OF: IMMERSING A FLAT FERROUS BASE STRIP, WITH EACH SURFACE FREE OF OXIDES AND OTHER IMPURITIES AND HAVING A STRIP TEMPERATURE BELOW 900*F., INTO A BATH OF MOLTEN ZINC CONTAINING TRACES OF ALUMINUM; COATING EACH SURFACE OF SAID BASE STRIP WITH THE SAME THICKNESS OF SAID ZINC, BETWEEN 0.1 AND 1.0 OUNCE PER SQUARE FOOT OF SAID ZINC; CONTROLLING THE VARIATION IN SAID ZINC COATING ON A SURFACE, FROM SIDE TO SIDE AND FROM FRONT EDGE TO REAR EDGE OF THE STRIP, TO $.05 OUNCE PER SQUARE FOOT; HEATING THE COATED STRIP, AT A STRIP HEATING RATE OF 25*30*F. PER SECOND, TO A MAXIMUM STRIP TEMPERATURE BETWEEN 1000*-1100*F. TO PROMOTE THE DIFFUSION OF IRON FROM SAID FERROUS BASE STRIP INTO SAID COATING; COOLING SAID STRIP IMMEDIATELY UPON ATTAINMENT OF SAID MAXIMUM STRIP TEMPERATURE, WITHOUT HOLDING AT SAID MAXIMUM STRIP TEMPERATURE, TO A STRIP TEMPERATURE OF 800*F.; CONTROLLING THE COOLING RATE, BETWEEN SAID MAXIMUM STRIP TEMPERATURE AND 800*F., TO DIFFUSE IRON FROM SAID FLAT FERROUS BASE STRIP INTO THE COATING IN AN AMOUNT OF IRON BETWEEN 4 AND 7% OF THE FINAL COATING WEIGHT; AND THEN COOLING SAID STRIP UPON ATTAINMENT OF SAID 800*F. TO A TEMPERATURE BELOW 800*F. AT A COOLING RATE SUFFICIENT TO PREVENT FURTHER DIFFUSION OF IRON INTO SAID COATING. 